Measuring systems are known, for use on numerical control machines, for determining the position and/or the dimensions of mechanical workpieces, for example by means of contact detecting probes, mounted on the machine, that wireless transmit signals to remote receiver units, displace, in the course of the measuring cycles, with respect to the workpiece, contact the surface to be checked by means of a stylus, and, in consequence of the contact, modify some features of the wireless transmitted signals. Each receiver unit is connected in its turn, by means of an interface unit, with the associated numerical control unit that, by processing other signals relating to the spatial position of the probe, achieves information about the position of the workpiece surfaces.
Probes can be equipped with electric batteries for the power supply of contact detecting circuits and transmission devices. The wireless transmission can occur, for example, by sending electromagnetic signals of optical or radio-frequency type.
A contact detecting probe including a wireless infrared transmission system is disclosed in patent U.S. Pat. No. 4,509,266, according to which the information about the contact between the stylus of the probe and a workpiece is represented by a shift in the frequency of the optical signal transmitted.
In order to identify with a suitable amount of accuracy the instant when contact occurs, it is necessary that the delays--that necessarily arise in the course of the transmission of the state of the probe--be sufficiently short and repeatable.
A method for achieving a good repeatability, in systems of digital type, is that of utilizing a serial transmission of asynchronous type: in this case a message, consisting of a definite bit sequence, is immediately sent, in an asynchronous way with respect to the sequence of the serial signal, the moment when the probe detects the change of state. The information indicating the exact moment when there occurs the change in the state of the probe is implicitly given by the message end.
However, since for the asynchronous transmission there must be employed a receiver that is extremely quick in synchronising with the received signal, such method is rather sensitive to noise. Especially in the case of radio-frequency transmission, a synchronous transmission is undoubtedly preferable since, owing to the fact that it is possible to extract in the receiver a signal of synchronism of remarkable stability, it assures a major immunity against noise.
Every transmission of electromagnetic signals of synchronous type sets the need for a time resolution given by the bit period, so necessarily introducing an inaccuracy, of an equal entity, in the delay time.
In order to minimize the aforementioned inaccuracy, one could envisage the use of a sufficiently high bit rate, in other terms bit periods as short as possible. However, this would mean the employing of broad transmission bandwidths and their use may not be in compliance with the radio communication regulations, or limiting the total number of available channels.
It should also be considered that, for safety reasons, in the course of the operation of contact detecting probes with wireless transmission systems, the transmitter on board of the probe periodically sends to the receiver, connected to the machine control unit, a message of state that, in addition to confirming the current state of the probe, enables to test the operation of the transmission system. In the event that, as a periodical message of state is being transmitted, there is a change in the state of the probe, there may occur malfunctions owing to the fact that, while it is necessary to wait for the end of the message in the course of transmission, before sending the new message containing the change of state information, a non repeatable casual delay is added. A method for overcoming this inconvenience consists in inserting in any case, before starting the transmission of a message containing the information regarding the change of state, a fixed delay time that is sufficient for ensuring that any possible message in the course of transmission has ended. Nevertheless, it is obvious that this method, even though ensuring repeatability, increases the overall transmission time.